Method for manufacturing a semiconductor film

ABSTRACT

A method for manufacturing a semiconductor film includes a step of preparing a first member including a semiconductor substrate, a semiconductor layer, and a separation layer provided between the semiconductor substrate and the semiconductor layer, a step of bonding or attracting a second member which is hardly heated by induction heating, onto the semiconductor layer of the first member, and a step of separating semiconductor layer from the semiconductor substrate at the separation layer by heating the semiconductor substrate by induction heating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing asemiconductor film, and more particularly, to a method for manufacturinga semiconductor film which is suitable for use as a solar cell or asilicon-on-insulator (SOI) substrate.

2. Description of the Related Art

Recently, a technique for manufacturing an SOI substrate has beenattracting notice as a technique for improving the processing speed of asemiconductor device and allowing saving of electric power. The SOIsubstrate is obtained by forming a semiconductor film having a thicknessof about several tens of nm to a few μm, for example, a single-crystalsilicon film, on an insulating layer. Methods for inexpensivelymanufacturing a solar cell using such a thin semiconductor film havealso been proposed.

U.S. Pat. No. 5,856,229 describes a method for manufacturing such an SOIsubstrate. In this method, first, a first substrate (wafer) comprisingnonporous single-crystal silicon is prepared, and a porous silicon layeris formed by anodizing a surface of the first substrate. Then, anonporous single-crystal silicon layer is formed on the porous siliconlayer according to epitaxial growth. An insulating layer comprisingsilicon oxide is formed by oxidizing the surface of the nonporoussingle-crystal silicon layer, and a multilayer structure is formed bybonding a second substrate on the surface of the insulating layer. Then,an SOI substrate is manufactured by separating the nonporoussingle-crystal silicon layer from the first substrate at the poroussilicon layer by applying an external force, such as a tensile force orthe like, to the multilayer structure, and transferring the nonporoussingle-crystal silicon layer onto the second substrate via theinsulating layer.

U.S. Pat. No. 6,054,363 describes another method for manufacturing anSOI substrate. In this method, the same processing as described above isperformed until a multilayer structure is formed. After this processing,the nonporous single-crystal silicon layer is separated from the firstsubstrate by applying an abrupt thermal stress to the porous siliconlayer, by heating the nonporous single-crystal silicon layer by causinga current to flow only therein.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method formanufacturing a semiconductor film more simply and efficiently byimproving the above-described conventional methods.

According to one aspect of the present invention, a method formanufacturing a semiconductor film includes the steps of preparing afirst member including a semiconductor substrate, a semiconductor layer,and a separation layer provided between the semiconductor substrate andthe semiconductor layer, bonding or attracting a second member which ishardly heated by induction heating, onto the semiconductor layer of thefirst member, and separating the semiconductor layer from thesemiconductor substrate at the separation layer by heating thesemiconductor substrate by induction heating.

According to another aspect of the present invention, a method formanufacturing a semiconductor film includes the steps of preparing afirst member including a semiconductor substrate, a semiconductor layer,and a separation layer provided between the semiconductor substrate andthe semiconductor layer, bonding or attracting a second member whoseresistivity is higher than a resistivity of the semiconductor substrate,onto the semiconductor layer of the first member, and separating thesemiconductor layer from the semiconductor substrate at the separationlayer by heating the semiconductor substrate by induction heating.

According to still another aspect of the present invention, a method formanufacturing a semiconductor film includes the steps of preparing afirst member including a semiconductor substrate, a semiconductor layerwhose resistivity is higher than a resistivity of the semiconductorsubstrate, and a separation layer provided between the semiconductorsubstrate and the semiconductor layer, and separating the semiconductorlayer from the semiconductor substrate at the separation layer byheating the first member by induction heating. It is desirable that theresistivity of the semiconductor layer is at least 10 times theresistivity of the semiconductor substrate. It is desirable that theresistivity of the semiconductor layer is at least 1 Ω·cm, and theresistivity of the semiconductor substrate is equal to or less than 0.1Ω·cm.

In the present invention, the first member is prepared by a step offorming a porous silicon layer, serving as a separation layer, byanodizing a surface of a nonporous silicon substrate, and a step offorming a nonporous silicon layer on the porous silicon layer accordingto epitaxial growth. (3) The first member may also be prepared by a stepof forming an ion-implanted layer, serving as a separation layer, exceptfor a silicon layer where ions are not implanted on a surface thereof,by implanting at least one type of ions selected from hydrogen, nitrogenand helium to a predetermined depth from a surface of a siliconsubstrate. In this process, a protective film may be formed on thesurface of the silicon substrate before implanting the ions.

In the present invention, the step of heating the semiconductorsubstrate by induction heating is performed by mounting the bonded orattracted first and second members on an induction-heating mount aroundwhich a coil is wound, and causing a current to flow in thesemiconductor substrate by supplying the coil with a high-frequencycurrent. Slits may be formed in the separation layer before heating thesemiconductor substrate by induction heating. A tensile force, acompressive force or a shearing force may be applied simultaneously withthe induction heating. A pressure or a hydrostatic pressure by a fluidmay be applied to the separation layer simultaneously with the inductionheating. The second member may be cooled simultaneously with theinduction heating.

After separating the semiconductor layer, a residue of the separationlayer remaining on the semiconductor layer is removed according toetching, if necessary. After separating the semiconductor layer, aremaining semiconductor substrate may be reutilized for preparinganother first member. At that time, a residue of the separation layerremaining on the semiconductor substrate may be removed according toetching, if necessary.

According to yet another aspect of the present invention, a method formanufacturing a solar cell includes the steps of forming a poroussilicon layer by anodizing a surface of a p⁺-type nonporous siliconsubstrate, sequentially forming a p⁻-type nonporous silicon layer and ann⁺-type nonporous silicon layer on the porous silicon layer according toepitaxial growth, attracting an attraction mount which is hardly heatedby induction heating, on the n⁺-type nonporous silicon layer, separatingthe p⁻-type and n⁺-type nonporous silicon layers from the p⁺-typenonporous silicon substrate at the porous silicon layer by heating thep⁺-type nonporous silicon substrate by induction heating, and formingelectrodes on the separated p⁻-type and n⁺-type nonporous silicon layer.

According to yet a further aspect of the present invention, a method formanufacturing an SOI substrate includes the steps of forming a poroussilicon layer by anodizing a surface of a p⁺-type nonporous siliconsubstrate, forming a p⁻-type nonporous silicon layer on the poroussilicon layer according to epitaxial growth, forming a silicon-oxidelayer on a surface of the p⁻-type nonporous silicon layer, forming amultilayer structure by bonding another nonporous silicon substrate on asurface of the silicon-oxide layer, and separating the p⁻-type nonporoussilicon layer from the p⁺-type nonporous silicon substrate at the poroussilicon layer by heating the multilayer structure by induction heating.

The foregoing and other objects, advantages and features of the presentinvention will become more apparent from the following description ofthe preferred embodiments taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a first embodimentof the present invention;

FIGS. 2A-2F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a second embodimentof the present invention;

FIGS. 3A-3F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a third embodimentof the present invention;

FIGS. 4A-4F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a fourth embodimentof the present invention;

FIGS. 5A-5G are schematic cross-sectional views illustrating a methodfor manufacturing a solar cell according to the present invention; and

FIGS. 6A-6G are schematic cross-sectional views illustrating a methodfor manufacturing an SOI substrate according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A-1F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a first embodimentof the present invention. In order to manufacture a semiconductor film,first, as shown in FIG. 1A, a nonporous single-crystal silicon substrate101 is prepared as a semiconductor substrate. A low-resistivity materialwhich can be heated by induction heating, such as p⁺-type silicon orn⁺-type silicon, is desirable as the nonporous single-crystal siliconsubstrate 101.

Then, as shown in FIG. 1B, a porous silicon layer 102 is formed on asurface of the nonporous single-crystal silicon substrate 101 byanodizing the nonporous single-crystal silicon substrate 101. The poroussilicon layer 102 operates as a separation layer. A hydrogen-fluoride(HF) solution or a solution obtained by mixing alcohol with thissolution may be used as an anodization solution. At that time, bychanging the anodization solution or the current density duringanodization, the porous silicon layer 102 may be formed in a multilayerstructure in which a plurality of layers having different porosities arelaminated in the direction of the thickness. By thus providing a layerhaving a high porosity in a part of the multilayer structure, it is easyto perform separation or to control a portion to be separated.

Then, as shown in FIG. 1C, a nonporous single-crystal silicon layer 103is formed on the porous silicon layer 102 according to epitaxial growth.The nonporous single-crystal silicon layer 103 may be formed accordingto chemical vapor deposition (CVD), liquid deposition or the like. Ap⁻-type silicon layer whose resistivity is higher than the resistivityof the nonporous single-crystal silicon substrate 101 is desirable asthe nonporous single-crystal silicon layer 103. The nonporoussingle-crystal silicon layer 103 may include a plurality of layershaving different conduction types or compositions. The above-describedstructure in which the nonporous single-crystal silicon layer 103 isformed on the nonporous single-crystal silicon substrate 101 via theporous silicon layer 102 operates as a first member 104.

Before forming the nonporous single-crystal silicon layer 103 in theabove-described manner, an oxide film may be formed on the inner wallsof pores in the porous silicon layer 102, followed by annealing in areductive atmosphere including hydrogen. By such annealing, siliconatoms on the surface of the porous silicon layer 102 move to reduce thesizes of the pores. As a result, defects in the nonporous single-crystalsilicon layer 103 growing on the porous silicon layer 102 can bereduced. The oxide film is formed in advance on the inner walls of poresin order to prevent silicon atoms from moving within the pores to closethe pores. As a result, when removing a residue of the porous siliconlayer 102 by etching as will be described later, the residue can be moreeasily removed.

A method for forming oxide films on both of the inner walls of pores andthe surface of the porous silicon layer 102 by performing heat treatmentof the porous silicon layer 102, for example, in an oxygen atmospheremay be used as the method for forming an oxide film only in the innerwalls of pores of the porous silicon layer 102 as described above.Thereafter, by processing the surface of the porous silicon layer 102with a hydrogen-fluoride (HF) solution, only the oxide layer on thesurface of the porous silicon layer 102 can be removed while leaving theoxide film on the inner walls of pores.

Then, as shown in FIG. 1D, after attracting the surface of the nonporoussingle-crystal silicon layer 103 of the first member 104 onto anattraction mount 105, the first member 104 attracted on the attractionmount 105 is mounted on an induction-heating mount 106. A heating coil107 is wound around the induction-heating mount 106. A high-frequencycurrent is caused to flow in the heating coil from an AC power supply108. The attraction mount 105 is made of a material which is hardlyheated by induction heating, i.e., a material which is substantially notheated by induction heating. A high-resistivity material, such asalumina, Photoveel (the trade name of a product made by Sumikin CeramicsCo., Ltd.), Macor (the trade name of a product made by CorningIncorporated) or the like, may preferably be used as such a material.That is, the attraction mount 105 operates as a second member. Althoughin the first embodiment, the silicon substrate 101 faces the inductionheating mount 106, the attraction mount 105 may face theinduction-heating mount 106.

The attraction mount 105 is provided in order to generate a temperaturedifference with respect to the silicon substrate 101. Accordingly, theattraction mount 105 has a resistivity higher than the resistivity ofthe silicon substrate 101. The temperature difference with respect tothe silicon substrate 101 may be increased by providing a pipe (notshown) within the attraction mount 105 and causing water, or coolednitrogen gas, helium gas or the like to flow in the pipe, i.e., byproviding a cooling mechanism within the attraction mount 105.

Instead of the above-described attraction mount 105, a member to bebonded on the surface of the nonporous single-crystal silicon layer 103may also be used. For example, when manufacturing an SOI substrate, thenonporous single-crystal silicon layer 103 is bonded on a supportingsubstrate before being separated from the silicon substrate 101. In thiscase, the supporting member operates as the second member. Asingle-crystal silicon substrate manufactured according to a Czochralski(CZ) method, a single-crystal silicon substrate manufactured accordingto a floating-zone (FZ) method, a single-crystal silicon substratesubjected to hydrogen annealing, a transparent glass substrate, or thelike may be used as the supporting member.

When using the supporting member as the second member in theabove-described manner, the resistivity of the supporting member must behigher than the resistivity of the silicon substrate 101. Furthermore,the supporting member is preferably made of a material which issubstantially not heated by induction heating, i.e., a material which ishardly heated by induction heating. It is desirable that the resistivityof the supporting member is at least 1 Ω·cm, preferably, at least 10Ω·cm, and more preferably, at least 100 Ω·cm.

When using a silicon substrate as the supporting substrate, thesupporting member may be bonded on nonporous single-crystal layer 103via an insulating layer. At that time, the insulating layer may beformed on the surface of the nonporous single-crystal silicon layer 103,or may be formed on both the surface of the nonporous single-crystalsilicon layer 103 and the surface of the silicon substrate. For example,silicon-oxide layers formed by performing thermal oxidation of thesurface of the nonporous single-crystal silicon layer 103 and thesurface of the silicon substrate are used as such insulating layers.

Then, as shown in FIG. 1E, the silicon substrate 101 is heated byinduction heating by causing a high-frequency current to flow in theheating coil 107 wound around the induction-heating mount 106 from theAC power supply 108. At that time, since the attraction mount 105 ishardly heated, a temperature difference is provided between the siliconsubstrate 101 and the attraction mount 105. Although it depends on theporosity of the porous silicon layer 102, the thermal conductivity ofthe porous silicon layer 102 is usually lower than the thermalconductivity of the silicon substrate 101, and a temperaturedistribution (difference or gradient) is produced starting from theporous silicon layer 102, i.e., the separation layer. Due to thistemperature difference, a thermal stress is applied to the poroussilicon layer 102. As a result, cracks are produced in the poroussilicon layer 102, resulting in separation of the nonporoussingle-crystal silicon layer 103 from the silicon substrate 101. Thatis, the nonporous single-crystal silicon layer 103 is separated from thesilicon substrate at the porous silicon layer 102. In order to realizesuch separation, the above-described temperature difference is desirablyat least 500° C.

Before performing induction heating as shown in FIG. 1E, slits may beprovided at sides of the porous silicon layer 102. Furthermore, atensile force, a compressive force or a shearing force may be applied tothe porous silicon layer 102 by a suitable expedient as a separationassist simultaneously with the induction heating. Furthermore, apressure or a hydrostatic pressure exerted by a fluid may also beapplied to the porous silicon layer 102 simultaneously with theinduction heating. Since the structure of the porous silicon layer 102is more fragile than the silicon substrate 101 and the nonporoussingle-crystal silicon layer 103, separation can be accelerated byapplying such an external force.

As shown in FIG. 1E, a residue 102 a of the porous silicon layer 102sometimes remains on the separated nonporous single-crystal siliconlayer 103. In such a case, the residue 102 a may be removed by etching,if necessary. In addition, a residue 102 b of the porous silicon layer102 also sometimes remains on the silicon substrate 101 from which thenonporous single-crystal silicon layer 103 has been separated. Such aresidue 102 b may also be removed by etching, if necessary.

Thus, as shown in FIG. 1F, the nonporous single-crystal silicon layer103 and the silicon substrate 101 are obtained. The nonporoussingle-crystal silicon layer 103 is used, for example, for manufacturinga semiconductor device, such as a solar cell or the like. On the otherhand, the silicon substrate 101 can be reutilized for preparing anotherfirst member. That is, by using the silicon substrate 101 shown in FIG.1F, another nonporous single-crystal silicon layer 103 can again bemanufactured according to the processing shown in FIGS. 1A-1E.

The principle of induction heating will now be briefly described. Asubstance to be heated, comprising a metal or a low-resistivitymaterial, is disposed within a winding made of a conductive pipe (mainlymade of copper) called a heating coil. By causing a high-frequencycurrent to flow in the heating coil, high-frequency magnetic fluxes aregenerated to cause an eddy current to flow in the substance to beheated, and the temperature rises due to the Joule heating. Thisoperation is called induction heating, and has features such that, forexample, rapid heating can be performed, the running cost is low, andlocalized heating can be performed.

When performing induction heating in the present invention, selectiveheating of the silicon substrate 101 is an important point. Accordingly,the resistivity of the silicon substrate 101 is preferably equal to orless than 0.1 Ω·cm, and more preferably, equal to or less than 0.05Ω·cm. In order to provide an effective temperature difference, it isdesirable that the resistivity of the nonporous single-crystal siliconlayer 103 is at least 1 Ω·cm. However, in the first embodiment, thenonporous single-crystal silicon layer 103 is attracted or bonded on theattraction mount 106, which is hardly heated by induction heating, andthe heat of the nonporous single-crystal silicon substrate 103 istransmitted to the attraction mount 106. Hence, the nonporoussingle-crystal silicon layer 103 need not always have theabove-described high resistivity. That is, although it has beendescribed that the nonporous single-crystal silicon layer 103 desirablycomprises p⁻-type silicon whose resistivity is higher than theresistivity of the silicon substrate 101, the nonporous single-crystalsilicon layer 103 may comprise non-doped silicon, p⁺-type silicon,n⁻-type silicon or n⁺-type silicon in accordance with the desiredsemiconductor film.

FIGS. 2A-2F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a second embodimentof the present invention. In FIGS. 2A-2F, the same components as thosein FIGS. 1A-1F are indicated by the same reference numerals, and furtherdescription thereof will be omitted.

In the second embodiment, first, as shown in FIG. 2A, a nonporoussingle-crystal silicon substrate 201 is prepared. Then, as shown in FIG.2B, ions 209 of at least one type selected from rare gases, such ashydrogen, nitrogen, helium and the like, are implanted to apredetermined depth from the surface of the silicon substrate 201. Atthat time, before implanting ions, it is preferable to form a protectivelayer comprising a silicon-oxide layer or the like on the surface of thesilicon substrate 201.

By the ion implantation, as shown in FIG. 2C, an ion-implanted layer 202is formed except for a silicon layer 203 where ions are not implanted onthe surface thereof. The ion-implanted layer 202 operates as aseparation layer. A structure in which the silicon layer 203 is formedon the silicon substrate 201 via the ion-implanted layer 202 in theabove-described manner serves as a first member 204.

Then, as shown in FIG. 2D, after attracting the surface of the siliconlayer 203 of the first member 204 onto an attraction mount 105, thefirst member 204 attracted on the attraction mount 105 is mounted on aninduction-heating mount 106. Then, as shown in FIG. 2E, the siliconsubstrate 201 is heated by induction heating by causing a high-frequencycurrent to flow in a heating coil 107 wound around the induction-heatingmount 106 from an AC power supply 108. Since defects and distortion areconcentrated in the ion-implanted layer 202, very small bubblesagglomerate under the effect of heating at 400-600° C. On the otherhand, since the attraction mount 105 is hardly heated, a temperaturedifference is provided between the silicon substrate 101 and theattraction mount 105, and a temperature distribution is providedstarting from the ion-implanted layer 202, i.e., the separation layer.Due to this temperature difference, a thermal stress is applied to theion-implanted layer 202. As a result, cracks are produced in theion-implanted layer 202, to separate the silicon layer 203 from thesilicon substrate 201.

As shown in FIG. 2E, a residue 202 a of the ion-implanted layer 202sometimes remain on the separated silicon layer 203. In such a case, theresidue 202 a may be removed by etching, if necessary. In addition, aresidue 202 b of the ion-implanted layer 202 also sometimes remains onthe silicon substrate 201 from which the silicon layer 203 has beenseparated. Such a residue 202 b may also be removed by etching, ifnecessary. The residue 202 a or 202 b may be removed not only byetching, but also, for example, by smoothing the surface of the siliconlayer 203 or the silicon substrate 201, respectively, by performingannealing after grinding the surface.

Thus, as shown in FIG. 2F, the silicon layer 203 and the siliconsubstrate 201 are obtained. The silicon layer 203 is used, for example,for manufacturing a semiconductor device, such as a solar cell or thelike. On the other hand, the silicon substrate 201 can be reutilized forpreparing another first member. That is, by using the silicon substrate201 shown in FIG. 2F, the silicon layer 203 can again be manufacturedaccording to the processing shown in FIGS. 2A-2E.

FIGS. 3A-3F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a third embodimentof the present invention. In the third embodiment, a semiconductor layeris separated from a semiconductor substrate by utilizing a difference inthe resistivity between the semiconductor substrate and thesemiconductor layer. In FIGS. 3A-3F, the same components as those inFIGS. 1A-1F are indicated by the same reference numerals, and furtherdescription thereof will be omitted.

In the third embodiment, first, as shown in FIG. 3A, a nonporoussingle-crystal silicon substrate 101 having a first resistivity isprepared as a semiconductor substrate. In order to be sufficientlyheated by induction heating, the first resistivity is preferably equalto or less than 0.1 Ω·cm, and more preferably, equal to or less than0.05 Ω·cm. A material comprising, for example, p⁺-type silicon orn⁺-type silicon, may be preferably used as the silicon substrate 101.

Then, as shown in FIG. 3B, a porous silicon layer 302 is formed on asurface of the nonporous single-crystal silicon substrate 101 byanodizing the nonporous single-crystal silicon substrate 301. The poroussilicon layer 302 operates as a separation layer.

Then, as shown in FIG. 3C, a nonporous single-crystal silicon layer 303having a second resistivity is formed on the porous silicon layer 302according to epitaxial growth. The above-described structure in whichthe nonporous single-crystal silicon layer 303 is formed on the siliconsubstrate 301 via the porous silicon layer 202 operates as a firstmember 304.

The second resistivity is higher than the first resistivity, i.e., theresistivity of the silicon substrate 301. The second resistivity ispreferably at least 10 times, and more preferably, at least 100 timeshigher than the first resistivity. It is desirable that the secondresistivity is at least 1 Ω·cm. More specifically, the nonporoussingle-crystal silicon layer 303 comprises, for example, non-dopedsilicon, p⁻-type silicon or n⁻-type silicon. In this description,p⁺-type silicon or n⁺-type silicon has an impurity concentration equalto or more than 10¹⁷ atoms/cm³, and p⁻-type silicon or n⁻-type siliconhas an impurity concentration equal to or less than 10¹⁶ atoms/cm³.Usually, the resistivity of p⁺-type silicon or n⁺-type silicon is atleast 0.1 Ω·cm, and the resistivity of p⁻-type silicon or n⁻-typesilicon is at least 1 Ω·cm.

Then, as shown in FIG. 3D, the first member 304 is mounted on aninduction heating mount 106. Although in the third embodiment, thesilicon substrate 101 faces the induction-heating mount 106, thenonporous single-crystal silicon layer 303 may face theinduction-heating mount 106.

Then, as shown in FIG. 3E, the silicon substrate 101 is heated byinduction heating by causing a high-frequency current to flow in aheating coil 107 wound around the induction-heating mount 106 from an ACpower supply 108. At that time, since the resistivity of the nonporoussingle-crystal silicon layer 303 is higher than the resistivity of thesilicon substrate 301, the silicon substrate 301 is selectively heated.As a result, a temperature difference is provided between the nonporoussingle-crystal silicon layer 303 and the silicon substrate 101. Due tothis temperature difference, a thermal stress is applied to the poroussilicon layer 302. As a result, cracks are produced in the poroussilicon layer 302, to separate the nonporous single-crystal siliconlayer 103 from the silicon substrate 101. In the third embodiment, also,as in the first embodiment, a tensile force, a compressive force, ashearing force, or a pressure or a hydrostatic pressure by a fluid mayalso be applied to the porous silicon layer 102 as separation assistingmeans.

As shown in FIG. 3E, a residue 302 a of the porous silicon layer 302sometimes remains on the separated nonporous single-crystal siliconlayer 303. In addition, a residue 202 b of the porous silicon layer 302also sometimes remains on the silicon substrate 301 from which thenonporous single-crystal silicon layer 303 has been separated. Such aresidue 202 a or 202 b may also be removed by etching, if necessary, asin the first embodiment.

Thus, as shown in FIG. 3F, the nonporous single-crystal silicon layer303 and the silicon substrate 301 are obtained. The nonporoussingle-crystal silicon layer 303 is used, for example, for manufacturinga semiconductor device, such as a solar cell or the like. On the otherhand, the silicon substrate 301 can be reutilized for preparing anotherfirst member, as in the first embodiment. That is, by using the siliconsubstrate 301 shown in FIG. 3F, another nonporous single-crystal siliconlayer 303 can again be manufactured according to the processing shown inFIGS. 3A-3E.

FIGS. 4A-4F are schematic cross-sectional views illustrating a methodfor manufacturing a semiconductor film according to a fourth embodimentof the present invention. In FIGS. 4A-4F, the same components as thosein FIGS. 1A-1F are indicated by the same reference numerals, and furtherdescription thereof will be omitted.

In the fourth embodiment, first, as shown in FIG. 4A, a nonporoussingle-crystal silicon substrate 401 having a first resistivity isprepared. Then, a nonporous single-crystal silicon layer 408 having asecond resistivity is formed on the silicon substrate 401 according toepitaxial growth. The first and second resistivities are set in the samemanner as in the third embodiment. A substrate comprising p⁺-typesilicon may, for example, be used as the silicon substrate 401, and ap⁻-type silicon layer may, for example, be used as the silicon layer408. The silicon layer 408 is formed according to CVD or liquiddeposition.

Then, as shown in FIG. 4B, ions 409 of at least one type selected fromrare gases, such as hydrogen, nitrogen, helium and the like, areimplanted to a predetermined depth from the surface of the silicon layer408. It is preferable to implant ions such that the projected range,i.e., the region where the concentration distribution of implanted ionsis highest, is present within the silicon layer 408 or at the interfacebetween the silicon substrate 401 and the silicon layer 408. On theother hand, when intending to provide a back surface field (BSF) effectin the thin film after separation, ions may be implanted such that theregion where the concentration distribution of implanted ions is highestis present within the silicon substrate 401, using a p⁺-type or n⁺-typesilicon substrate. Before implanting ions, it is preferable to form aprotective layer comprising a silicon-oxide layer or the like, on thesurface of the silicon substrate 408.

By the ion implantation, as shown in FIG. 4C, an ion-implanted layer 402is formed except for a silicon layer 403 where ions are not implanted onthe surface. The ion-implanted layer 402 operates as a separation layer.A structure in which the silicon layer 403 is formed on the siliconsubstrate 401 via the ion-implanted layer 402 in the above-describedmanner serves as a first member 404.

Then, as shown in FIG. 4D, the first member 404 is mounted on aninduction-heating mount 106. Although in the fourth embodiment, thesilicon substrate 401 faces the induction-heating mount 106, the siliconlayer 403 may face the induction-heating mount 106.

Then, as shown in FIG. 4E, the silicon substrate 401 is heated byinduction heating by causing a high-frequency current to flow in aheating coil 107 wound around the induction-heating mount 106 from an ACpower supply 108. Since defects and distortion are concentrated in theion-implanted layer 402, very small bubbles agglomerate by heating at400-600° C. On the other hand, the silicon layer 403 is hardly heatedbecause the resistivity of the silicon layer 403 is higher than thesilicon substrate 401. As a result, a temperature difference is providedbetween the silicon substrate 401 and the silicon layer 403, and atemperature distribution is provided starting from the ion-implantedlayer 402, i.e., the separation layer. Due to this temperaturedifference, a thermal stress is applied to the ion-implanted layer 402.As a result, cracks are produced in the ion-implanted layer 402, toseparate the silicon layer 403 from the silicon substrate 401.

Residues 402 a and 402 b of the ion-implanted layer 402 sometimes remainon the separated silicon layer 403, and the silicon substrate 401 fromwhich the silicon layer 403 has been separated, respectively. As in thesecond embodiment, the residues 402 a and 402 b may be removed byetching, or by smoothing the surfaces of the silicon layer 403 and thesilicon substrate 401 by performing annealing after grinding thesurfaces.

Thus, as shown in FIG. 4F, the silicon layer 403 and the siliconsubstrate 401 are obtained. The silicon layer 403 is used, for example,for manufacturing a semiconductor device, such as a solar cell or thelike. On the other hand, the silicon substrate 401 can be reutilized forpreparing another first member, as in the first embodiment. That is, byusing the silicon substrate 401 shown in FIG. 4F, another silicon layer403 can again be manufactured according to the processing shown in FIGS.4A-4E.

Although in the above-described third and fourth embodiments, thesilicon layer is separated by utilizing the difference in theresistivity between the silicon substrate and the silicon layer, asecond member whose resistivity is higher than the resistivity of thesilicon substrate may also be used together. That is, after attractingthe attraction mount 105 described in the first embodiment onto thesurface of the silicon layer 303 or 403, the silicon substrate may beheated according to induction heating.

Alternatively, a multilayer structure may be provided by bonding thesilicon layer 303 or 403 onto a supporting substrate directly or via aninsulating layer, and the multilayer structure may be heated accordingto induction heating. In this case, the supporting substrate operates asthe second member. The silicon substrate or the glass substratedescribed in the first embodiment may be used as the supportingsubstrate. When bonding the silicon layer onto the supporting member viathe insulating layer, the insulating layer may be formed according to amethod similar to the above-described method.

Although in the above-described first through fourth embodiments, thenonporous single-crystal silicon substrate and the nonporoussingle-crystal silicon layer are used as the semiconductor substrate andthe semiconductor layer, respectively, the semiconductor substrate andthe semiconductor layer may be formed using any other appropriatematerials, provided that a separation layer can be formed.

EXAMPLE 1

A semiconductor film was formed according to the method shown in FIGS.3A-3F. First, as shown in FIG. 3A, a p⁺-type nonporous single-crystalsilicon substrate (silicon wafer) 301 having a resistivity of 0.02 Ω·cmand a diameter of 3 inches was prepared. This silicon substrate 301 wasimmersed in a solution obtained by mixing a hydrogen-fluoride (HF)solution and ethanol. After causing a current having a current densityof 7 mA/cm² to flow in the silicon substrate 301 for one minute, acurrent having a current density of 20 mA/cm² was caused to flow in thesilicon substrate 301 for ten minutes, to form the porous silicon layer302 shown in FIG. 3B.

Then, the silicon substrate 301 was placed within a CVD apparatus, andannealing was performed at 950° C. by introducing hydrogen gas into theapparatus, to smooth the surface of the porous silicon layer 302. Then,by introducing a source gas into the CVD apparatus, a p⁻-type nonporoussingle-crystal silicon layer 303 was formed on the porous silicon layer302 according to epitaxial growth, to form the first member 304 shown inFIG. 3C. The resistivity of the formed silicon layer 303 measured usinga monitor was 1.5 Ω·cm.

Then, as shown in FIG. 3D, the first member 304 was mounted on theinduction-heating mount 106, and a current having a frequency of 350 kHzand an output of 2 kW was caused to flow in the heating coil 107 fromthe AC power supply 108. The silicon substrate 301 was thereby heated to500° C. in 20 seconds. As a result, a shearing force was generated dueto the temperature difference between the silicon substrate 301 and thesilicon layer 303, and, as shown in FIG. 3E, the silicon layer 303 wasseparated from the silicon substrate 301 at the porous silicon layer302.

By immersing the separated silicon layer 303 in a solution obtained bymixing a hydrogen-fluoride (HF) solution, a hydrogen-peroxide (H₂O₂)solution, ethanol and water, a residue 302 a of the porous silicon layer302 remaining on the silicon layer 303 was removed by etching, to obtainthe semiconductor layer shown in FIG. 3F, i.e., the nonporoussingle-crystal silicon layer 303. By also removing a residue 302 b ofthe porous silicon layer 302 remaining on the silicon substrate 301 byetching, the silicon substrate 301 having a smooth surface shown in FIG.3F was obtained. This silicon substrate 301 could be again used formanufacturing another silicon layer 303 according to the processingshown in FIGS. 3A-3F.

EXAMPLE 2

A solar cell was manufactured according to a method to be described withreference to the schematic cross-sectional views shown in FIGS. 5A-5G.In FIGS. 5A-5G, the same components as those shown in FIGS. 1A-1F areindicated by the same reference numerals, and further descriptionthereof will be omitted.

First, as shown in FIG. 5A, a p⁺-type nonporous single-crystal siliconsubstrate (silicon wafer) 501 having a resistivity of 0.01 Ω·cm and adiameter of 4 inches was prepared. This silicon substrate 501 wasimmersed in a solution obtained by mixing a hydrogen-fluoride (HF)solution and ethanol. After causing a current having a current densityof 8 mA/cm² to flow in the silicon substrate 301 for one minute, acurrent having a current density of 20 mA/cm² was caused to flow in thesilicon substrate 501 for ten minutes, to form a porous silicon layer502 shown in FIG. 5B. The porous silicon layer 502 included two porouslayers having different porosities.

Then, the silicon substrate 501 was annealed in a hydrogen atmosphere tosmooth the surface of the porous silicon layer 502. Then, a p⁻-typenonporous single-crystal silicon layer 503 having a thickness of 50 μmand an n⁻-type nonporous single-crystal silicon layer 505 having athickness of 0.2 μm were sequentially formed on the porous silicon layer502 according to liquid deposition, to provide a first member 504 shownin FIG. 5C.

Then, as shown in FIG. 5D, after attracting the silicon layer 505 of thefirst member 504 onto an attraction mount 105, the first member 504 wasmounted on an induction-heating mount 106. The attraction mount 105 hada cooling mechanism for causing cooled nitrogen gas to flow within amounted pipe.

Then, the silicon substrate 501 was selectively heated by causing acurrent having a frequency of 500 kHz and an output of 5 kW to flow in aheating coil 107 from an AC power supply 108. At the same time, thesilicon layers 505 and 503 were cooled by the cooling mechanism of theattraction mount 105. The temperature difference between the siliconsubstrate 501, and the silicon layers 505 and 503 reached 500° C. in 10seconds. As a result, the porous silicon layer 502 was destructed by ashearing force generated due to a difference in thermal expansion at theporous silicon layer 502, and, as shown in FIG. 5E, the silicon layers505 and 503 were separated from the silicon substrate 501.

By immersing the separated silicon layers 505 and 503 in a solutionobtained by mixing a hydrogen-fluoride (HF) solution, ahydrogen-peroxide (H₂O₂) solution, ethanol and water, a residue 502 a ofthe porous silicon layer 502 remaining on the silicon layer 503 wasremoved by etching, to obtain a semiconductor layer shown in FIG. 5F,i.e., a laminated structure comprising the p⁻-type silicon layer 503 andthe n⁻-type silicon layer 505 was obtained. By also removing a residue502 b of the porous silicon layer 502 remaining on the silicon substrate501 by etching, a silicon substrate 501 having a smooth surface shown inFIG. 5F was obtained. This silicon substrate 501 could be again used formanufacturing another solar cell according to the processing shown inFIGS. 5A-5F.

As shown in FIG. 5G, by performing heat welding of the p⁻-type siliconlayer 503 of the laminated structure obtained in the above-describedmanner onto an aluminum plate 506, serving as an electrode as well as asupporting member, and simultaneously diffusing aluminum into thep⁻-type silicon layer 503, a p⁺-type silicon layer 507 was formed. Then,by forming an antireflection layer 509 after forming current collectingelectrodes 508 on the n⁻-type silicon layer 505, a thin-film solar cellshown in FIG. 5G was manufactured.

Although in Example 2, the residue 502 a on the p⁻-type silicon layer503 was removed, it is only necessary to perform such processing ifnecessary. Such processing may not be performed if a solar cell can bemanufactured even if the residue 502 a remains.

EXAMPLE 3

An SOI substrate was manufactured according to a method to be describedwith reference to the schematic cross-sectional views shown in FIGS.6A-6G. In FIGS. 6A-6G, the same components as those shown in FIGS. 1A-1Fare indicated by the same reference numerals, and further descriptionthereof will be omitted.

First, as shown in FIG. 6A, a p⁺-type nonporous single-crystal siliconsubstrate (silicon wafer) 601 having a resistivity of 0.01 Ω·cm and adiameter of 5 inches was prepared. This silicon substrate 601 wasimmersed in a solution obtained by mixing a hydrogen-fluoride (HF)solution and ethanol. After causing a current having a current densityof 7 mA/cm² to flow in the silicon substrate 601 for five minutes, acurrent having a current density of 30 mA/cm² was caused to flow in thesilicon substrate 601 for ten minutes, to form a porous silicon layer602 having a thickness of 5 μm shown in FIG. 6B. The porous siliconlayer 602 included two porous layers having different porosities.

Then, by heating the silicon substrate 601 on which the porous siliconlayer 602 was formed in an oxygen atmosphere at 400° C. for one hour, ansilicon-oxide film was formed on the inner walls of pores and thesurface of the porous silicon layer 602. Then, only the silicon-oxidelayer on the surface of the porous silicon layer 602 was removed byprocessing the surface of the porous silicon layer 602 with a hydrogenfluoride (HF) solution.

The silicon substrate 601 was then placed within a CVD apparatus, andannealing was performed at 950° C. by introducing hydrogen gas into theapparatus, to smooth the surface of the porous silicon layer 602. Byintroducing a source gas into the CVD apparatus, a p-type nonporoussingle-crystal silicon layer 603 having a thickness of 0.3 μm was formedon the porous silicon layer 602 according to epitaxial growth. Theresistivity of the formed silicon layer 603 measured using a monitor was10 Ω·cm. Then, the surface of the silicon layer 603 was thermallyoxidized to form a silicon-oxide layer 605 having a thickness of 100 nmas an insulating layer. Thus, a first member 604 shown in FIG. 6C wasformed.

Then, after activating the surface of the silicon-oxide layer 605 byprojecting nitrogen (N₂) plasma thereon, the silicon-oxide layer 605 wastightly superposed on a separately prepared silicon substrate (siliconwafer) 606, as shown in FIG. 6D. The first member 604 and the siliconsubstrate 606 were bonded together by performing heat treatment of theintegrated structure at 600° C. for three hours, to obtain a laminatedstructure 607.

Then, as shown in FIG. 6E, after attracting the silicon substrate 606 ofthe laminated structure 607 onto an attraction mount 105 having awater-cooling mechanism, the laminated structure 607 was mounted on aninduction-heating mount 106. Then, the silicon substrate 601 wasselectively heated by causing a current having a frequency of 700 kHzand an output of 10 kW to flow in a heating coil 107 from an AC powersupply 108. At the same time, the silicon substrate 606 was cooled bythe cooling mechanism of the attraction mount 105. The temperaturedifference between the silicon substrate 601 and the silicon layer 603reached 550° C. in 20 seconds. As a result, the porous silicon layer 602was destructed by a shearing force generated due to a difference inthermal expansion at the porous silicon layer 602, and, as shown in FIG.6F, the silicon layer 603 was separated from the silicon substrate 601,and was transferred on the silicon substrate 606 via the silicon-oxidelayer 605.

By immersing the silicon substrate 606, on which the silicon layer 603was transferred, in a solution obtained by mixing a hydrogen-fluoride(HF) solution, a hydrogen-peroxide (H₂O₂) solution, ethanol and water, aresidue 602 a of the porous silicon layer 602 remaining on the siliconlayer 603 was removed by etching. Then, by annealing the siliconsubstrate 606 on which the silicon layer 603 was transferred in areductive atmosphere including hydrogen at 1,100° C. for one hour, thesurface of the silicon layer 603 was smoothed. Thus, as shown in FIG.6G, an SOI substrate 608 having the single-crystal silicon layer 603 onthe silicon substrate 606 via the silicon-oxide layer 605 wasmanufactured.

By also removing a residue 602 b of the porous silicon layer 602remaining on the silicon substrate 601 by etching, the silicon substrate601 having a smooth surface shown in FIG. 6G was obtained. This siliconsubstrate 601 could be again used for manufacturing another SOIsubstrate according to the processing shown in FIGS. 6A-6G.

EXAMPLE 4

A solar cell was manufactured according to another method to bedescribed below. In the description the same components as those shownin FIGS. 1A-1F are indicated by the same name, and further descriptionthereof will be omitted.

First, a p⁺-type nonporous single-crystal silicon substrate (siliconwafer) having a resistivity of 0.01 Ω·cm and a diameter of 4 inches wasprepared. Then, a p⁻-type nonporous single-crystal silicon layer havinga resistivity of 2 Ω·cm and a thickness of 1.2 μm and an n⁺-typenonporous single-crystal silicon layer having a thickness of 0.2 μm weresequentially formed on the silicon substrate according to epitaxialgrowth by CVD. Then, hydrogen ions with energy of 450 eV were implantedfrom the side of the n⁺-type silicon layer to a concentration of7.0×10¹⁶/cm², to form an ion-implanted layer to a depth of about 2 μmfrom the surface of then +-type silicon layer. Then, a first member, inwhich the ion-implanted layer, a surface p⁺layer of the siliconsubstrate where ions were not implanted, the p⁻-type silicon layer andthe n⁺-type silicon layer are sequentially laminated, was formed on aremaining portion of the silicon substrate.

Then, after attracting the n⁺-type silicon layer of the first memberonto an attraction mount 105 having a cooling mechanism, the firstmember was mounted on an induction-heating mount 106. The attractionmount 105 had a cooling mechanism for causing cooled nitrogen gas toflow within a mounted pipe. Then, the silicon substrate was selectivelyheated by causing a current having a frequency of 450 kHz and an outputof 3 kW to flow in a heating coil 107 from an AC power supply 108, whilecooling the n⁺-type silicon layer, the p⁻-type silicon layer, and thesurface p⁺layer of the silicon substrate by the cooling mechanism of theattraction mount 105. As a result, very small bubbles agglomeratedwithin the ion-implanted layer by the heating, and the temperaturedifference between the remaining portion of the silicon substrate, andthe n⁺-type silicon layer, the p⁻-type silicon layer and the surfacep⁺layer reached 500° C. in 10 seconds. As a result, cracks were producedin the ion-implanted layer by a shearing force generated due to adifference in thermal expansion at the ion-implanted layer, and, then⁺-type silicon layer, the p⁻-type silicon layer and the surface p⁺layerwere separated from the remaining portion of the silicon substrate.

By removing a residue of the ion-implanted layer from the separatedsilicon layers by etching, a semiconductor film having a laminatedstructure comprising the p⁺layer, the p⁻-type silicon layer and then⁺-type silicon layer was obtained. By also removing a residue of theion-implanted layer remaining on the remaining silicon substrate byetching, a silicon substrate having a smooth surface was obtained. Thissilicon substrate could be again used for manufacturing another solarcell according to the above-described processing.

By coating a conductive adhesive on the p⁺-type silicon layer of thelaminated structure obtained in the above-described manner, thelaminated structure was bonded on a supporting substrate made ofstainless steel. Then, by forming an antireflection layer after formingcurrent collecting electrodes on the n⁺-type silicon layer, a thin-filmsolar cell was manufactured. In this solar cell, the back surface (BSF)effect was obtained due to the p⁺-type silicon layer.

The individual components shown in outline in the drawings are all wellknown in the semiconductor-film manufacturing arts, and their specificconstruction and operation are not critical to the operation or the bestmode for carrying out the invention.

While the present invention has been described with respect to what arepresently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. To the contrary, the present invention is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

What is claimed is:
 1. A method for manufacturing a semiconductor filmcomprising the steps of: preparing a first member including asemiconductor substrate, a semiconductor layer whose resistivity ishigher than a resistivity of the semiconductor substrate, and aseparation layer provided between the semiconductor substrate and thesemiconductor layer; and separating the semiconductor layer from thesemiconductor substrate at the separation layer by heating the firstmember by induction heating, wherein said step for preparing the firstmember comprises a step of forming a porous silicon layer, serving as aseparation layer, by anodizing a surface of a p⁺-type nonporous siliconsubstrate, and a step of forming a p⁻-type nonporous silicon layer onthe porous silicon layer according to epitaxial growth.
 2. A method formanufacturing a semiconductor film comprising the steps of: preparing afirst member including a semiconductor substrate, a semiconductor layerwhose resistivity is higher than a resistivity of the semiconductorsubstrate, and a separation layer provided between the semiconductorsubstrate and the semiconductor layer; and separating the semiconductorlayer from the semiconductor substrate at the separation layer byheating the first member by induction heating, wherein said step forpreparing the first member comprises a step of forming a p⁻-type siliconlayer on a p⁺-type silicon substrate according to epitaxial growth, andforming an ion-implanted layer, serving as a separation layer, exceptfor a p⁻-type silicon layer where ions are not implanted on a surfacethereof, by implanting at least one type of ions selected from hydrogen,nitrogen and helium to a predetermined depth from a surface of thep⁻-type silicon layer.
 3. A method according to claim 2, wherein saidstep of preparing the first member further comprises a step of forming aprotective film on the surface of the p⁻-type silicon layer beforeimplanting the ions.